In a conventional flat knitting machine a sinker between knitting needles is employed to control a yarn threaded through a knitting needle in loop releasing, loop coupling, loop escaping and loop forming processes. The sinker has a protrusive edge at one side corresponding to the knitting needle. When the sinker is rotated the protrusive edge presses the yarn picked up by the knitting needle to form a loop, thereby to form a pattern on the surface of the knitting fabric.
In order to stabilize rotation of the sinker, a sinker bed usually is provided on the needle bed. For instance, European patent EP1522618 and EP1522617 disclose a sinker bed mounted onto a needle bed via a coupling seat. The sinker bed has a trough corresponding to each sinker to restrict movements of the sinker without skewing. In addition, the sinker bed has a control portion to control rotation of the sinker, such as that disclosed in U.S. Pat. No. 5,475,990. In general, the sinker control portion includes a first cam and a second cam positioned next to each other. The first cam and second cam are braced by a linked movement mechanism on a base on which the sinker bed is movable horizontally. The linked movement mechanism has at least one magnet to form magnetic attraction with the sinker bed. When the sinker control portion moves horizontally relative to the sinker bed, the horizontal direction of movement determines whether the first cam and second cam will push the sinker on the first cam and second cam, then change the rotation angle to form a yarn loop at a different size.
In practice, whether the sinker control portion can move steadily and horizontally relative to the sinker bed determines whether the sinker can accurately perform the rotation movement. If the surface of the sinker bed is rough, unsmooth movement of the sinker control portion could be happen, and rotation control of the sinker could be faulty. Hence the sinker bed usually is a steel bar integrally formed at a length substantially same as the needle bed. One side of the sinker bed corresponding to the sinker control portion has to be formed with a very smooth surface. Precise fabrication is required that makes the resulting product very expensive.